1. Field of the Invention
The invention relates to a hybrid motor vehicle. More specifically, the invention relates to a hybrid motor vehicle that includes an engine that outputs motive power for running the vehicle, an electric motor that takes in and outputs motive power for running the vehicle, and a battery that gives electric power to and receives electric power from the electric motor.
2. Description of Related Art
This type of hybrid motor vehicle usually is equipped with an engine, a first electric motor, a power distribution integration mechanism of which a ring gear, a carrier and a sun gear are connected to a drive shaft linked to an axle, an output shaft of the engine and a rotating shaft of the first electric motor, a second electric motor whose rotating shaft is connected to the drive shaft, and a battery that gives electric power to and receives electric power from the first electric motor and the second electric motor. In a related-art hybrid motor vehicle, if during stop of operation of the engine, the required power of the vehicle based on required torque of the drive shaft becomes increases to or above a first threshold value, the engine and the first and second electric motors are controlled so that the required torque is output to the drive shaft while the required power is output from the engine until the required power decreases to or below a second threshold value that is smaller than the first threshold value, and if during operation of the engine, the required power decreases to or below the second threshold value, the engine, the first electric motor and the second electric motor are controlled so that the required torque is output to the drive shaft while the engine is in a stopped state, until the required power increase to or above the first threshold value (see, e.g., Japanese Patent Application Publication No. 2007-131103 (JP 2007-131103 A)). In this hybrid motor vehicle, by setting as a first threshold value a value close to a lower-limit value at which the engine can be comparatively efficiently operated, intermittent operation of the engine is executed so as to achieve good efficiency of the vehicle as a whole.
In this hybrid motor vehicle, the magnitude of the charge/discharge electric power of the battery is likely to be greater when the engine and the first and second electric motors are controlled so that the required torque is output to the drive shaft while operation of the engine is in a stopped state than when the engine and the first and second electric motors are controlled so that the required torque is output to the drive shaft while the engine is outputting the required power. If the battery is charged or discharged with great electric power when the battery has degraded to a certain degree, there is a risk of further progress of the degradation of the battery, although this problem does not occur when the battery has not degraded.